Since the advent of the sharp rise in fuel prices and the possibility of fuel shortages, a number of conservation measures have been undertaken. Certain of the conservation ideas for gaseous fuels have been mandated by state legislatures. It has been common practice for many years to use a very simple, inexpensive and reliable pilot arrangement for gas fired equipment. In the past it has been common to use a standing pilot, that is one that continuously burns and is monitored by a flame sensing device, such as a thermocouple. This type of a system has proved to be very inexpensive, and reliable. Many state legislatures have now mandated that the standing pilot is not in the country's best interest in fuel conservation, and the standing pilot must be replaced with some other type of fuel ignition arrangement.
One type of fuel ignition arrangement that is coming into prominence is a system normally referred to as a direct spark ignition system. In this type of system an electric spark is generated across a gap to ignite a gaseous fuel as it emanates from a gas burner. This type of arrangement, while it appears to be simple and straight forward, creates some very serious safety problems. Firstly, there is a problem of properly igniting a fuel; secondly, there is a problem of a gas valve failure which would allow for the continuous flow of raw fuel into a burner when none was required. This can be not only wasteful, but very hazardous. In order to alleviate the hazard in a direct spark ignition type of system, it has become common, and even required, that two gas valves be placed in series so that the failure of one valve will not preclude the closing of the fuel flow channel by the second valve. This type of arrangement is generally referred to as the redundant valve arrangement.
Where the valves are controlled electronically, an additional problem is created in that electronic components may failure in modes which may cause an unsafe condition in a direct spark ignition system. Also, there is a problem of possible welding of relay contacts which may be used to control the electric current to the fuel valve. Any direct spark ignition system for control of fuel flow valves must also take into consideration the failure modes of the electronic components and, therefore must be designed so that any component failure causes a shut down of the fuel flow. This is considered to be a safe mode of failure.